So about 1/3 of bottom should have heat although I go about 1/2. This gives you the temp ranges that allow the snake to thermo regulate. The belly heat comes from the UTH. In a tub, depending on room temps and drafts, etc it my be hard to get decent ambient temps. I have my Boa in a 40qt bin using a UTH set at 90 and a second for the cool side at 80 and see ambient temps at about 80. Tubs really maintain humidity well but heat is tricky at least for me. I have to keep the room temp at 75 minimum.
I am looking into ways to insulate the tub and keeping a minimal layer of substrate helps also. Sterility or Rubbermaid are popular and there is a sticky going over tub set up that is worth looking at.